1. Field of the Invention
The present invention generally relates to an electronic power module. More specifically, the invention relates to electronic power modules having a silicon nitride substrate for automotive applications.
2. Description of Related Art
Power modules including IGBT (insulated gate bipolar transistor), FREDs (fast recovery epitaxial diode), MOSFETs, and other semiconductor chips have been used in automotive applications for many years. The power modules must be able to operate at a high ambient temperature. In addition, the high power dissipation required by the power modules further increases the temperature variations affecting the components of the power module. With such large temperature swings, the difference in thermal expansion of the components in the power module often causes reliability issues. A major cause of reliability issues is due to warping of the substrate. Substrate warpage of even 250 microns across a die can present a problem. Typically, the printed solder paste thickness is only 125 microns thick. Therefore, a warpage of 250 microns would not allow the die to be placed flat on the solder print.
Many manufacturers have used smaller powered dies on multiple substrates to address the warpage problem. For example, four 50 Amp chips have been used on four separate substrates rather than a single 200 Amp chip on a single substrate as desired. However, using multiple substrates requires the use of a metal base plate which adds cost and complexity to the module. In addition, a thermal expansion mismatch between the die and substrate can also cause mechanical stress in the electrical connections thereby compromising reliability. Repetitive high power switching applications tend to increase temperature and shorten the life expectancy of solid-state power devices.
One application that subjects the power module to repetitive high power switching is the electric automobile application, especially when the electric vehicle is driven in a city where stopping at a red light every few minutes is typically required. The power required to accelerate a vehicle from a standing start is substantially greater than the power required to maintain a constant speed. Another example is an electric assisted automotive power steering system while in a parking maneuver. Quite often in this situation the tires come in contact with the curb. Tires contacting the curb drives the power assist system to provide maximum output until the controlling processor reacts to the “stall load” demand by reducing the amount of assist. Automotive applications typically require very high peak power demands followed by a short cool down in a repetitive cycle. To address the power dissipation problem caused by the frequent high power switching, power modules were designed with multiple chips and multiple substrates, as shown in FIG. 1.
FIG. 1 shows a IGBT module 10 with a first semiconductor chip 12 and a second semiconductor chip 26. The first semiconductor chip 12 is attached to a ceramic substrate 14. The ceramic substrate 14 includes a copper layer 16 that includes a circuit pattern (not shown). The first semiconductor chip 12 is attached to the copper layer 16 by wire bonds 20. In addition, substrate 14 has a copper layer 22 which can be used for grounding or thermal dissipation. The copper layer 22 is attached to a base plate 24. The base plate 24 acts as a foundation or support for all of the semiconductor components.
The second semiconductor chip 26 is attached to a separate second ceramic substrate 28. The second ceramic substrate 28 has a copper layer 30 which includes a circuit pattern (not shown). Second semiconductor chip 26 is electrically connected to the circuit pattern of copper layer 30 by wire bonds 32. The second substrate 28 also has a copper layer 34 for grounding, mechanical balancing, and dissipating heat. Copper layer 34 is also attached to base plate 24. Attached to the circuit pattern on copper layers 16, 30 are leads 42, 44, 46, 48 for connecting the IGBT module with other devices or circuits external of module 10. To improve the thermal dissipation and protect the components of module 10 a silicone gel 40 is disposed over the semiconductor chips 12, 26 to cover and protect them. In addition, an epoxy resin 38 is disposed on top of silicone gel 40 to further protect and seal module 10. The epoxy resin case 36 is attached to the base plate 24 and provides additional structural protection for the components of IGBT module 10.
The multiple ceramic substrates 14 and 28 help to mitigate thermal expansion problems. However, the multiple ceramic substrates also complicate the manufacture of module 10, add additional weight, and add significant cost to the module.
In view of the above, it is apparent that there exists a need for an improved power module for automotive applications that is easier to manufacture, weighs less, and has lower component costs.